Acquisitions & Divestitures Purchase Accounting Entries (Details) - 2016 Permian Basin Acquisition [Member] $ in Millions	Sep. 30, 2017 USD ($)
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	$ 591.0
Proved Property [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	406.2
Unproved Property [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	214.2
Asset Retirement Obligation Costs [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	(11.6)
Bargain Purchase Gain [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	$ (17.8)